Interpretive Summary: This paper has shown that the Mexico nitrogen index can be used as new technology for preserving water quality in La Laguna. The tool can be used to evaluate how the management practices entered into the index by the user affect the estimated nitrogen uptake by forage crops from this region (r2 = 0.55; P<0.001), usually taking only a few minutes. The tool can also be used to analyze the potential effect of applying given quantities of manure and/or nitrogen fertilizers on the amount of residual inorganic nitrogen (r2 = 0.85; P<0.001).
Our analysis of over 20 studies found a strong correlation between the excessive nitrogen inputs of manure and fertilizer that are traditionally used across the area and a larger potential risk for large quantities of nitrate to leach out of the root zone of these irrigated forage crops (r2 = 0.84; P<0.001). A considerable number of the analyzed studies (10) had residual inorganic nitrate in the soil after the forage harvest greater than 300 kg N ha-1. Additionally, about seven studies had a risk potential of nitrate leaching higher than 150 kg N ha-1. Almost all of the studies had total nitrogen uptakes lower than 300 kg N ha-1, yet the traditional nitrogen application rate is higher than needed for desired productivity levels and creates a large risk of negative effects on the environment.
The results of these studies are in agreement with studies from Martinez et al. (2006) and Cueto et al. (2005) that found a correlation between intensive forage management systems with increased nitrates in groundwater in this dairy region of Mexico. Our analysis of the studies strongly suggests that with certain changes in management, intensive dairy systems can be both productive and environmentally sustainable, without applying such large quantities of nitrogen. A tool like the Mexico nitrogen index can potentially assist crop managers, nutrient managers, and educators in determining the potential effects of high nitrogen inputs on nitrogen losses, and in sharing this information with farmers in the region.

Technical Abstract:
Though intensive dairy systems, such as the one established in the Comarca Lagunera region in Mexico, help societies maintain a reliable supply of food products for the expanding population of humans, the high applications of manure and fertilizer traditionally applied by these operations in this region can result in reactive nitrogen entering water resources as nitrate. Our objective was to test and adapt the new nitrogen index to evaluate the impacts of the intensive dairy system used in Comarca Lagunera, which has nearly half a million cattle, making it one of the largest concentrations of dairy operations in North America. Data from various studies on the effects of manure and fertilizer rates on soil inorganic nitrogen and nitrogen uptake by forage crops were used to calibrate and validate the nitrogen index newly adapted for Mexico. The Mexico nitrogen index successfully assessed nitrogen uptake and conducted a risk assessment of the effect of manure and nitrogen fertilizers on residual inorganic nitrogen (P<0.001), showing a strong correlative relationship between the high nitrogen inputs that are traditionally used across Comarca Laguna and greater potential risk for nitrate leaching. The study suggests that if management practices in this region remain unchanged and do not begin to incorporate nitrogen budgets, the risk of leaching nitrates to local groundwater will continue to increase. The study also demonstrates that tools like the Mexico nitrogen index can be used to assess the potential use of practicable management alternatives to current practices in intensive dairy systems in this region.